Several types of vehicles, including semi-trailer truck vehicles and the like, have multiple sets of axle/wheel suspension assemblies arranged in tandem so as to adequately support relatively heavy loads. To adjust the load support provided by these tandem assemblies, it is known to employ suspension systems utilizing adjustably pressurized air springs.
When the vehicle is carrying a relatively light load, it is desirable to relieve the load transmitting relationship between the vehicle and one or more of the tandem axle/wheel assemblies, and also to disengage the axle/wheel assembly from ground contact so as to reduce tire wear. To relieve load support when an air suspension system is employed, air pressure can be reduced. To achieve disengagement of the tires from the ground surface, devices commonly referred to as axle lift mechanisms can be employed. Prior lift mechanisms utilized stressed mechanical springs acting directly between a vehicle frame and the axle. When the downward load forces exerted on the axle by the suspension system were relieved, such as through deflation of air springs, lifting forces exerted by the mechanical springs pulled the axle upwardly to a raised position.
The foregoing type of lift mechanism has several disadvantages. For example, with the axle in a raised position, sufficient spring force must be maintained to support the axle and various components of the suspension system. Correspondingly, the substantially increased spring force when the axle is in the lowered position comprises preload forces on the suspension system, thereby reducing the actual maximum vehicle payload carried by the suspension system.
A type of known axle lifting mechanism is depicted as lift mechanism 60 shown in FIG. 1. Lift mechanism 60 is used with a trailing arm 62 pivotably mounted at one end to a bracket 66 through pivot connection 64. Bracket 66 is rigidly secured to a support frame 68 of a load-carrying vehicle. Although not shown in FIG. 1, the trailing arm 62 can also be connected to a vehicle axle and releasably coupled in a load supporting relationship to vehicle frame 68 through an air suspension system in a manner well known to those in the vehicle suspension trade.
The lift mechanism 60 includes a lever arm 70 having its lower end coupled to the vehicle frame 68 through a pivot connection 72. A rigid forged bar 74 is pivotably coupled at one end to a top portion of the lever arm 70 in an over-center arrangement through pivot connection 75. An opposing end of forged bar 74 is received through an aperture of a spring cup 76 and secured thereto with nut 78, washer 80 and bushing 82.
The spring cup 76 and forged bar 74 are mounted within a tubular housing 84. A compression spring 86 is also linearly mounted within the housing 84. One end of spring 86 bears against spring cup 76, while the other end of spring 86 bears against a stationary lip 77 of housing 84. Forces exerted on forged bar 74 by spring 86 can be varied by adjusting the axial location of threaded nut 78 along bar 74.
Also coupled to lever arm 70 and forged bar 74 at the pivot connection 75 is a bar link 90. The bar link 90 is correspondingly connected to a clevis link 92 and lifting chain 98 comprising a series of links 94. The lifting chain 98 is connected at its lower end to the trailing arm 62 through anchor 96.
The operation of the axle lift mechanism 60 is as follows: With the air spring (not shown) in a deflated state, the lift mechanism 60 will operate to lift the trailing arm 62 and interconnected axle and wheels. The lifting force is provided by forces exerted by the compression spring 86 bearing against spring cup 76. The forces exerted on spring cup 76 are translated through the forged bar 74 to rotate the lever arm 70 through pivot connection 72 to the position illustrated in FIG. 1. Rotation of lever arm 70 correspondingly exerts lifting forces on trailing arm 62 through bar link 90, clevis link 92 and lifting chain 98.
In the raised position, the lever arm 70 and other components of lift mechanism 60 will have the relative positions shown in FIG. 1. When the air spring is adequately inflated, forces exerted thereby on the trailing arm 62 overcome the forces exerted by compression spring 86, and the trailing arm 62 moves to a lowered position, thereby rotating the lever arm 70. As the lever arm 70 pivots in a clockwise direction as viewed in FIG. 1, the lever-type mechanical "lifting advantage" is decreased. That is, although the forces exerted by the compression spring 86 increase as lever arm 70 pivots, the lifting forces on trailing arm 62 will not substantially increase.
In normal use of the suspension the rotation of the trailing arm 76, whether by lifting or lowering the axle, or by oscillation due to road vibration, the spring cup 76 rubs against the housing 84. An unpleasant squeaking noise results. Further, the spring cup, housing and spring 86 are subject to unacceptable wear. Thus, the spring cup, housing and spring may have to be frequently replaced.